1. Field of the Invention
The present invention relates to an image heating apparatus for heating an image formed on a recording material by using an electrophotographic system or an electrostatic recording system. Such an image heating apparatus can be used, for example, as a fixing apparatus in a copier, a printer, a facsimile or the like.
2. Related Background Art
In image forming apparatuses such as electrophotographic copiers, printers or facsimiles, after an electrostatic latent image is formed on a photosensitive member, the electrostatic latent image is developed as a toner image which is then fixed onto a paper by thermally fusing and pressurizing the toner image. A fixing apparatus for fixing the toner image onto a recording material (paper or recording medium) is incorporated in such an image forming apparatus.
Japanese Patent Application Laid-open No. H06-318001 (1994) discloses a fixing apparatus of belt type (belt fixing apparatus). This belt fixing apparatus comprises an endless fixing belt, a fixing roller, a heating roller and a pressure roller. The heating roller and the fixing roller are disposed in parallel with each other and the fixing belt extends between and is wound around the heating roller and the fixing roller. The pressure roller is disposed in parallel with the fixing roller with the interposition of the fixing belt therebetween and is biased toward the fixing roller. As a result, pressure is generated between the fixing roller and the fixing belt. When the pressure roller is rotated, the fixing belt wound around the heating roller and the fixing roller is driven. A main heating source is arranged within the heating roller and an auxiliary heating source is arranged within the pressure roller. A recording material carrying a toner image is passed through an area (nip portion) where a portion of the fixing belt is pinched between the pressure roller and the fixing roller. While the recording material being passed through the nip portion, heat is applied to the recording material from the fixing belt and pressure is applied to the recording material from the pressure roller. In this case, the recording material is also subjected to auxiliary heat from the pressure roller. By the heat and the pressure applied to the recording material being passed through the nip portion, the toner image or non-fixed image is fixed onto the recording material.
Further, Japanese Patent Application Laid-open No. H04-44075 (1992) proposes a fixing apparatus of belt heating type for performing the heating via a belt having small heat capacity, as a system (on-demand system) having a high heat transferring efficiency and fast rising-up of the apparatus. In this apparatus, a heat resistive resin belt (fixing belt) as a heating rotary member is closely urged against heating means (heating heater) by a pressurizing rotary member (elastic) roller and is slidingly conveyed thereby, and a recording material as a heated material carrying a non-fixed image is introduced into a nip portion defined by the heating means and the pressurizing member with the interposition of the belt so that the recording material is conveyed together with the heat resistive resin belt. In this way, the non-fixed image is fixed onto the recording material as a permanent image by heat from the heating means via the belt and a pressurizing force of the nip portion.
In recent development of colorization of electrophotographic techniques, it has been requested for the fixing member contacted with the recording material to provide soft fixing. In the conventional fixing apparatuses of belt heating type, it is pointed out that, due to rigidity of a surface layer comprised of mold releasing resin such as fluororesin, toner particles may be crushed thereby to deteriorate a resolving power of the image and that, during color image fixing, since plural color layers are laminated, poor color mixing may occur. Provision of an elastic belt in which an elastic layer is provided between a base layer and a surface layer as means for solving these problems has been proposed in Japanese Patent Application Laid-open No. H10-321352 (1998).
In view of recent energy saving, it is known to provide a method in which consumption of electric power of the heating means is reduced by increasing thermal conductivity of the belt base layer by using metal such as nickel as the base layer of the belt in place of the heat resistive resin and to provide a technique in which a fixing property is further improved and further high speed can be achieved and uniformity of a temperature in a longitudinal direction of the nip portion is also enhanced, by increasing thermal conductivity of the belt base layer by using metal.
However, in the fixing apparatus of belt heating type, after the fixing operation, if the fixing apparatus is not used within a predetermined time period after the driving of the fixing belt is stopped, at a contact area between the fixing belt and the heating roller, the base layer and the elastic layer of the fixing belt are cooled and hardened in a condition that these layers are deformed. As a result that, when the fixing belt is used again, the fixing belt cannot be restored from the deformed condition quickly. Particularly when a diameter of the heating roller is reduced in order to achieve compactness of the apparatus, a winding mark (referred to as “winding curl” hereinafter) of the fixing belt is generated. Consequently, when the image is formed firstly in a day or when the image is formed after a long inoperative time, glossy unevenness and/or streak(s) is generated in the image due to such winding mark or a protruded portion is generated on the fixing belt due to the winding mark before the belt is introduced into the nip portion, with the result that poor image such as scattering of the image would be occurred during the fixing.
Further, in the above-mentioned fixing apparatus of belt heating type, curvatures of the rising-up portions of the belt at both ends of the nip portion are increased. As a result, if the apparatus is not used within the predetermined time period after the driving of the fixing belt is stopped after the fixing operation, bending marks may be formed on belt portions at the both ends of the nip portion having great curvature, with the result that, in response to such belt bending, glossy unevenness and stripe(s) may occur in the image.